Most electronic devices such as, for example, computers, televisions, radio receivers and amplifiers include electronic circuits formed by printed circuit boards. The art of printed circuit board fabrication which arose from the efforts of Strong et al., as disclosed in British Patent No. 690,691, has had a profound impact upon modern society. Ever since the initial efforts by Strong et al. it has been an ongoing goal of the printed circuit board industry to increase the number of electrical circuit components which can be provided on a given circuit board surface area. While pursuing this goal various new and improved methods of producing printed circuit boards have developed since the efforts of Strong et al. such as, for example, the meritorious methods disclosed in Whewell et al., U.S. Pat. No. 5,017,271 which is owned by Gould Inc.
In the manufacture of printed circuit boards, raw materials, including conductive foils, which are usually copper foils, and dielectric supports comprising organic resins and suitable reinforcements, are packaged together and processed under temperature and pressure conditions to produce products known as laminates. The laminates are then used in the manufacture of printed circuit boards. Generally, the laminates are processed by etching away portions of the conductive foil from the laminate surface to leave a distinct pattern of conductive lines and formed elements on the surface of the etched laminate. Laminates and/or laminate materials may then be packaged together with etched products to form multilayer circuit board packages. Additional processing, such as, for example, hole drilling and component attaching, will eventually complete the printed circuit board product.
The printed circuit board industry's push toward miniaturization and increased performance per package is resulting in conductors of ever smaller widths, more closely spaced on thinner substrates. The characteristics of the copper foil have a significant effect on the electrical performance of the finished printed circuit board. For example, a foil used in multilayer laminates must not crack during hole drilling. Also, foils which are less susceptible to wrinkling during the lamination process are preferable for reducing scrap losses. Similarly, and more importantly, variations in the thickness or surface texture of the copper foil will result in unpredictable electrical characteristics for any given printed circuit board.
Copper foils have been produced for printed circuit boards by two major methods, rolling or electrodeposition. The production of copper foil by electrodeposition processes involves the use of an electroforming cell (EFC) consisting of an anode and a cathode, an electrolyte bath solution, generally containing copper sulphate and sulphuric acid, and a source of current at a suitable potential. When voltage is applied between the anode and cathode, copper deposits on the cathode surface.
The process begins by forming the electrolyte solution, generally by dissolving (or digesting) a metallic copper feed stock in acid. After the copper is dissolved the solution is subjected to an intensive purification process to ensure that the electrodeposited foil contains no disruptions and/or discontinuities. Various agents for controlling the properties may be added to the solution.
The solution is pumped into the EFC and when voltage is applied between the anode and cathode, electrodeposition of copper occurs at the cathode. Typically, the process involves the use of rotatable cylindrical cathodes ("drum cathodes") that may be of various diameters and widths. The electrodeposited foil is then removed from the drum cathodes as a continuous web as the drum cathode rotates. Copper foils prepared using such conventional electrodeposition methodology have a smoother (drum) side and a rough or matte (copper deposit growth front) side.
In order to produce a foil of uniform thickness and surface finish, it is imperative that the plating surface of the drum cathode be uniform and consistent. More particularly, the quality and characteristics of the metal foils are a function of the quality of the plating surface of the drum cathode. In order to obtain a uniform matte finish the plating surface must exhibit uniform hardness and be free of surface defects and blemishes. Further, in order to maintain a suitable finish during use, the plating surface must exhibit a suitable degree of hardness.
Drum cathodes for use in producing copper metal foil have heretofore generally been manufactured using either stainless steel or ASTM Grade 7 titanium. ASTM Grade 7 titanium comprises 0.2 weight percent palladium (Pd). Titanium is generally preferred for in some applications it obviates certain environmental concerns. However, one major downside associated with a titanium anode is cost or unsatisfactory performance. More particularly, many prior art titanium drum cathodes are produced in a seamless or unitary form (i.e., a single continuous piece, no joining or welding) so as to afford a uniform matte finish along the outer diameter of the drum cathode. Unfortunately, the equipment which is required to form a seamless titanium drum cathode is quite expensive and the end result is a costly drum cathode. Alternatively, some prior art titanium drum cathodes are produced by forming a strip of titanium into a cylinder and welding together the ends. However, many of these drum cathodes do not provide a long lasting uniform matte finish free of defects and blemishes. More particularly, during use the weld zone begins to leave its mark or shadow on the metal foil thus necessitating more frequent replacement of such drum cathode as compared to a seamless drum cathode. Therefore, it is believed that if a method could be developed which would allow one to weld together a length .of titanium so as to provide a drum cathode that affords a long lasting uniform matte finish free of defects and blemishes, thereby eliminating the need for costly seamless production equipment and the expenses associated with the frequent replacement of prior art welded drum cathodes, considerable costs could be saved.